Ultrahigh-frequency bridge



Nov. 4, 1952 MORENO 2,616,951

ULTRAHIGH-FREQUENCY BRIDGE Filed Oct. 4, 1944 2 Sl-IEETSSHEET 1 SOURCE FIGJ INDICA INVENTOR TH EODORE' MORENO ATTORNEY Nov. 4, 1952 'r. MORENO 2,616,951

' ULTRAHIGH-FREQUENCY BRIDGE Filed 001;. 4, 1944 2 smears-swam z FIG.5

TRANSM RECEIVER INVENTOR THEODORE MORENO ATTORN EY Patented Nov; 4, 1952 UNITED STATES PATENT OFFICE I I ULTRAHIGPi-ZQ E ZELNCY BRIDG TheodoreMorenmGarden City, N. Y., assignor to The 'sperry'Corpjoration, a corporation of Delaware- Application 'cto'b'er4, 1944','Serial No. 557,182

9 Claims.

1' This inventionrel'ates to ultrahigh'and' micro wave frequency bridge circuits.

A bridge circuit, as is well known; isa circuit networkhaving several interconnected branches or arms which are-so arranged that whenan electromotive force is present in one branch, the response ofa suitable detecting device in another branch may be'madezero by a balancing adjustment" of the-electrical constants oi the network. Usually such circuits arecharacterized by the fact that if the detecting device and the electromotive force, are interchanged, with the "bridge circuit in itsbalanced "condition, the response of the detecting deviceis still zero;

The problem "inyall bridgesis-t'o -evolve acircuitapparatus having electrical constants which maybe adjusted to give a null output at'a de tector or'output arm-of thedevice with an input applied to'another arm.

In a low frequency s'tonebridge,the-electrical'constants of the bridge are considered to be concentrated at predeterminedpcints in the-form of "lumped impedances. The distributed constants-"inthecircuit areal-- most entirely due to the interconnecting conductors between these impedancesand, for all' pram tical purposes; are considered to have little e'ifec'i',

onithe operation of the bridge;

However, at higher frequencies; say about 10 million cycles 'per second, the errors'consequentto the presenceofeven very small stray inductances-andcapacitances or?"interconnecting con ductorslimit' the'useful frequency" range and flexibility-of the bridge considerably. Further extensionof theuse of. these conventional br-idg-e circuits to ultra high frequencies and beyond presents many diiiiculties.

cycles per second, :owing' tox'the various oomph-:-

cations and uncertainties which arise andwhich mount rapidly in importance witn increasing frequency 'ranget 7 As the frequency range increases, the-distributed constants of'ithe usual bridge circuit' become more! and m'ore significant in determining-theaction of the bridge or bridge circuit.- Forjex ample, as-appl'ied-t'o'the measurement of the impedances, the use of -these'--b'ridge circuits at ultra highfreduency" is adversely afrecre" the-impedance and calibration of' theihdicatih instrument, by theefict of the" 'leads to *the" bridge, such-'as the'Wheatauxiliary equipment, and similar "factors which vary with frequency so that theusefulirequency range of any one of these bridges or bridge circuits becomes rather limited. In addition, these conventional ultra high frequency circuits do not provide true equivalents of bridge circuits at extremely high frequencies in many applications where it is desirable to take advantage of-the characteristic of a bridge connection, since's'uch conventional circuits may not give true null indications or balanced outputs.

Known types of bridges offered for use at ultra high frequencies attempt by various expedients to overcome the deleterious effect of the distributed constantspresent in their bridge circuits or networks.- In these known ultra high frequency bridges, iour impedance elements are interconnected by finite lengths of transmission line in ring'fashion. One opposite pa'ir of elements serves as' balancing impedance elements. A third element serves as an input and is adapted to-be connect'ed tog-or-is'formedby asuitable high ne'cted'to acommon'pointand-does not have'in'- tercon'necti'ng transmission line elementssequen tially joining the impedance elements as in pi'ior forms of bridge-.- In a form adapted for" making' impedance measurements, neither the im pedance of the indicating: instrument norits calibration enter'sinto the precision of the measurement. Further, the input-impedance oi th'e present bridge has been found to -reniain siih stanti'ally" constant: with Wide variations-of "the: input frequency; r i

This improved bridge compri'ses an 'iinput arm to "which may' be connected 2; suitable eneratoror: source; two other arms knownv as'balancingl arms (since suitable impedance's o r loads are connected to them in order to balance the lir idgei) and a fourth arm-w liich servesasthe detector:or output arm. When'balanced, the input arid out"- put arms may be interchanged;

An important and-novel feature ofthe-present" bridge consists in the manner of connecting tlie input or generator arm of the bridge and the detector or output arm of the bridge in relation to the balancing arms of the bridge. If the input arm of the bridge is connected in shunt to the balancing arms, so that these arms are fed equal voltages in phase, then the output or detector arm is connected in series with the balancing arms so as not to respond to currents in the balancing arms that are equal and in phase. However, either one of the arms connected to the balancing arms may act as the input while the other then acts as the output.

One object of the invention is to provide an improved bridge circuit adapted to operate over a wide range of frequencies, and particularly at ultra high and microwave frequencies.

Another object is to provide an improved bridge circuit having a plurality of arms connected at a single point.

Another object is to provide an improved bridge circuit having a substantially constant input impedance characteristic with varying input frequencies.

Still another object is to provide an improved arrangement for interconnecting a plurality of circuits with one or more other circuits so as to prevent interaction among the said plurality of circuits.

Yet another object is to provide an improved bridge circuit adapted for making impedance measurements at ultra high frequencies and microwave frequencies.

Yet another object is to provide an improved bridge circuit of the character described, adapted for interconnection in systems of metal pipe Wave guide construction or coaxial line or parallel wire transmission line construction, or systems using any combination of these.

Other objects and advantages of the invention will become apparent as the description proceeds.

The invention will be more readily understood in the following description taken together with the accompanying drawings, in which:

Fig. 1 is a simple schematic diagram of an embodiment of the improved bridge, used for purposes of explaining the theory thereof;

Fig. 2 is a perspective view, with parts in cross section of a preferred embodiment of the improved bridge of the present invention having three coaxial line arms and a fourth arm in the form of a rectangular'wave guide;

Fig. 3 is a cross sectional elevational view of the preferred embodiment of the improved bridge of the present invention having three coaxial line arms and a fourth arm in the form of a rectangular wave guide;

Fig. 4 is a cross sectional elevational view showing a modification of the improved bridge structure illustrated in Fig. 3;

Fig. 5 is a schematic diagram illustrating an embodiment of the bridge according to the present. invention used as an isolating device between a high frequency transmitter and its receiver utilizing a common antenna;

Fig. 6 is aschematic diagram of another modification of the bridge of the present invention, having two wave guide balancing arms, a third wave guide arm and a fourth arm in the form of a coaxial line; q

Fig. 7. is a schematic diagram of a bridge according to the present invention, having two wave guide balancing arms, a third parallel two-wire line arm, and a fourth arm in the form of a coaxial line construction;

Fig. 8 is a schematic diagram of a modified bridge having two coaxial lines as balancing arms, a third coaxial line arm and a fourth balanced two-wire line arm which is looped for excitation; and

Fig. 9 is a schematic diagram of another modifled bridge having two coaxial line balancing arms, a coaxial line third arm, and a fourth .arm directly coupled to the other arms and being of balanced two-wire line construction.

The great majority of those who have attempted to overcome the difiiculties inherent in constructing high frequency bridges, especially in bridges designed for use at ultra high and microwave frequencies, have in most instances used portions of transmission line as interconnecting elements between the arms of their bridges. Although this has worked to a great extent in overcoming many of the attendant difficulties, the mere fact of having transmission lines interconnecting the arms of the bridges has presented a problem which has not heretofore been solved.

This problem results from the fact that these bridges have physical finite lengths of transmission lines interconnecting the bridge arms.

It is readily seen that although the physical length of the transmission line connecting these arms remains constant, the electrical characteristic of the transmission line will change with varying input frequency since their electrical lengths vary.

If, for example, an output load is in perfect bridge adjustment at a definite frequency, it receives no signal or energy. However, due to the finite lengths of interconnecting transmission line, if the input frequency varies, the electrical lengths of these transmission lines will in turn vary, resulting in a change in the input impedance of the bridge.

The present bridge overcomes this grave defeet by doing away with the ring-like interconnections between the arms of the bridge. The present ultra high frequency bridge is thus not appreciably influenced by a changing input frequency, but rather possesses substantially constant input impedance characteristics over an extremely wide band of frequencies. The essence of the bridge lies in-a novel construction having all the bridge arms connected at a common point. The input arm is connected in shunt with the arms carrying the balancing loads or impedances, and the output arm is connected in series with the arms carrying the balancing loads. However, either the shunt arm or series arm may act as the input or output to the bridge, the use of these arms being reciprocal in function. Further, the present bridge l-may utilize any form of ultra high frequency energy conductor to connect the bridge arms to their common junction, including wave guides, coaxial transmission lines or parallel-wire transmission lines. The balancing arms of the bridge are usually of the same type energy conductor.v

The input and output arms may use conductors of the same or different types, which may also be different from one another.

Referring to the drawings, Fig. 1 shows aschematic diagram of one form of bridge of the present invention useful in explaining its theory.

Arms 2 and 3 are the balancing arms, and are shown in the form of coaxial lines or sections connected at point 5 and having impedances-Zz and Z3 respectively connected at their outer ends. Arm 1 is the input arm and is shown as a simi- 5. la'r ecaxiai time -co uecting-a nigh frequency source Gtopoint-E. arm 4'is the output-arm and illustrated as a *wave guide having one end communicating with -the-coaxial lines-at point 5. and the other end connected to a suitable load or indicator 1. In thisway-source t 'isconnect'ed in shuntwith the balancing arms 2 and s,;-whi1e indicator or load 1 is connected in. series with arms 2 and =3. Thestructure-ofthese arms maytea's-shownm Figs. 2 and *S-described' below.-

simearms Zan'd -3-'are fed'in parallel by the source "6, the voltages existing across the input to these-armsat point t must be equal regardless ofthe impedances of arms 2 and3. But the current that; flows t n-arms 2 or 3 necessarily depends upon the ='re's'pe'ctive i-mpedances of these arms. "If the impedance-51mm Z3, respectively. or mesearms are-"not equal, there-will be. a cor--. responding inequality in the currents fiowing in the *elesed 'circuitsof arms 2- and.3-. Since arms rams-are coupled toarm: 41in series, so that arm 4 excited 1 bythe 'diiTe'rence inrcurrent flow. .in the balancing armed-2'. and. 3, anyinequalit-y. in current 'flow in :arms. 52. ziaindv 3.--.will :result in a voltage at arm I developed by theflow .ofcurre'nt across the impedancewof ;.arm 4. If. the impedances orcarms 2 iandifi areequal, there will be sno differentialcurrentflowing and no voltage will :appear at-arm The-circuit is then balanced in the sense that; an ordinary bridge "network is balanced, since a voltage applied to arm I. -of ,the bridge is incapable of producing any voltage in a secondarm 4 of the bridge. The source 6 and load 1 may, of course, be interchanged without affecting the bridge balance.

fl he bridge; just described has been found to havegan .inputrimpedance which is substantially constantrover a'wide. range of source. frequencies. Thisis believed due to the type of connection of the bridgearms. using one common junction .-point .5.

.Eig"s...2 and .3 illustrate a .physical form which the bridge schematically shown in Fig. '1 may assume. "The [inter-connecting conductors of emu-aid '3 mayjtake the form or asingle coaxial transmission 'ilin'e having an inner c'onductor t and an 'outerconductor 9. v

'Arm I is als f'o niedf'by a'coaxia'l'line having anouter conductor "I'l joined to conductor 9 at anopeni'ng inthe wall thereof. The inner conducto'r l2'of,arm "l is "connected directly to conductortfat point'fi.

' Arm *4'fisformed by a'recta'n'gular wave guide Itfhavin'g itsaxis perpendicular to coaxial line 8, Band with its narrow dimension extending "along line8,f9. Theend'of guide I 3 partially surrounds the outerwall of conductor 9, end plates 14, I4 beingused 'to complete the closure of guide I3. For coupling "guide 1'3 to the remainder of the device, a narrow slot I is:formed in the conductorjs permitting communication between .the guide I3'and the interior of conductor 9. This slot extends in "a plane perpendicular "to the axis of line "'8, '9, so "thatfits width extendsalong the direction of the electricfield inguide I3. In this way,-"the-fexcitation of guide I3 'through'slot I5 is responsive "to the displacement current flowing across the slot within conductor 9, which in turn"is-proportionalto"the difierence between the currents of arm's Zvand 3. Therefore guide I3 isjtrulyseries-connectedto arms 2 and 3.

Far tight coupling, t length of slot I5 should beiarge. "In the device of Figs. Ziand 3; "this length is limited by .the diameter :of lines 8; 9,

"which is determined usually by "the conventional 6 dimensions for such "lines. "To perml-t gr'eater length for slot I 5, recourse may be had to the construction of Fig. l-which is a slight modification of the bridge of Figs. 2-and 3.

In Fig. 4-, the portion of arms 2 and 3 nearthe junction oint 5 has anenlarged diameter formed by an enlarged sleeve I6 joined to conductor- 9 by suitable discs IT, IT.

This is a mechanical expedient allowing Ethe use of coaxial lines 8, 9 and II, 1 2 of usual dimens'ion, and yet maintaining the necessary slot dimensions for adequate couplihg'o'f the "wave guide to the coaxial line.

If it is desired to provide a coupling between the c'oaxialline 8, 9 and the wave guide 13 which has a'narrow band frequency response, theslot I5 may be dimensioned to be resonant atthe frequency of the wave-energy to be coupled therethrough.

Bridges of the type described above hav'e' many uses. One major use is in the measurement of impedances. For thispurpose, an impedanceto be measured is used as impedance Z2 of Fig. 1-. A standard, variable, calibrated impedance "is connected as Z3. With source 6 and indicator 1 connected as in Fig. 1, indicator 1 will givea null indication only when the calibrated impedance has'been adjusted 'to be equal to the-unknown impedance.

Fig. 5 illustrates another important application of the present bridge in excluding power-from one bridge arm while supplying powerto-another arm from a generator or transmitter for trans mission to a further arm. One of the balancing arms 3 is terminated ina suitable "antenna indicated schematically at 2'] for bothr'eceiv'ing and radiating. electromagnetic waves. The inputarm I isconnected'to a transmitter 22, while' the output'arm 4 is connected to a receiver 23. The transmitter 22 and receiver 23 may be interchanged. A dummy load 24 -is:connected to the other balancing arm v2; and is adjusted or-sele'cted to have an impedance value Ibalancing that of antenna 2|, so that the bridge is'balanced.

Since the bridge is properly. balanced, trans-.

mitted energy is not received; or felt by the receiver, but is radiated out into space from. the antenna 2|. The receivedenergyis received on the same antenna 21, and is fed to the receiver 23. In this way receiver 23 and "transmitter-'22 are completely isolated from one another, permitting use of a common antenna. This. arrangement may therefore be-used in duplexlcommunication systems or in echo-type object detecting and locating systems.

Many other uses of bridges ofthe present type are also possible. Thus, the impedance 22 con.- nected to the balancing arm 2 maybe replaced by a, rectifier crystal unit. The other balancing impedance Z3 may be a similar crystal or .a dummy balancing impedance of the same im'- pedance value. The input arm I may then'be connected to a suitable receiving antenna .or other source of high frequency wave, while the output armmay be connected to a local oscillator or other source of heterodyning frequency wave. The received wave impressed on arm I and the local oscillator or 'heterodyning wave impressed on arm 4 will thenbe mixed by the rectifier'connected to arm 2, so that an intermediate frequency output may be derived therefrom. If both arms 2 and 3 are connected to crystals, then two intermediate frequency'outputswill "be derived and maybesuitable combined. In each case, however, since the bridge *is in balanced condition; none of the received energy is lost to the local oscillator circuit connected to the output arm. In view'of this relative isolation of local oscillator and receiver circuits, the local oscillator requires much less power output and may be more tightly coupled to themixer circuit. It will be understood that such mixer circuits can be used apart from heterodyne receiver frequency wherever the mixing of two waves of the same or diiferent frequencies, with relative isolation between their circuits, is required. Obviously, the input and output arms may also be interchanged here. 1

While a preferred embodiment of the present invention has been described above, many other forms of bridges are possible, all having the common feature that the four bridge arms are connected at a common point. Thus, other forms of high frequency energy conductors may be substituted for the coaxial line or wave guide arms shown in the above Figs. 2 to 4.

Fig. 6 schematically illustrates a bridge circuit in which the balancing arms 2' and 3' are formed by wave guide sections 29 and 30 communicating at 28. The input arm I is also formed of a wave guide section 3|, and the output arm 4 is formed by a coaxial line section having inner conductor 26 and outer conductor 21. The inner conductor 26 extends within the space 28 formed by the junction of the wave guide arms 29, 30 and 3|, and thus acts in the nature of a probe antenna for coupling line 26, 21 to the wave guides. The line 26, 21 is thus effectively in shunt with the balancing arm wave guides 29 and 30, while wave guide 3| is in series with the balancing arms 29 and 30, as required.

Fig. 7 shows a further modification similar to Fig. 6, but in which the arm I is formed as a shielded two-wire transmission line, comprising a pair of parallel conductors 32 surrounded by shield 33. .Shield 33 is connected to the wave guide arms 29 and 30, while the conductors 32 extend within the junction space 28. Arm I is thereby still in effective series relation with the balancing arms 29 and 30.

Fig. 8 illustrates another embodiment of the invention having balancing arms 2 and 3 and output arm 4 in the form of T-connected coaxial lines 8, 9 and I I, I2 similar to Figs. 2 to 4. The input arm I, however, is formed as a. parallel two-wire transmission line having a pair of inner conductors 32 and an outer shield 33 which is connected to the outer conductors 9 and I I of the other arms 2, 3 and 4. The inner conductors 32 are connected together to form a loop 34 positioned within conductor 9 to be coupled with the magnetic field within the outer conductor 9 of the balancing arms 2 and 3. By virtue of the symmetric'configuration, loop 34 is again differentially responsive to the currents flowing in the balancing arms 2 and 3 and provides a series connection for the input arm I.

Fig. 9 shows a further embodiment of the present invention similar to Fig. 8, in which, however, the inner conductors 32 of the two-wire line forming arm I are directly connected to the inner conductor 8 of the balancing arms 2 and 3, equally displaced from the junction of the inner conductors 8 and I2 of arms 3 and 4. Functionally, this circuit operates in the same manner as that of Fig. 8.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof.- it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An ultra high frequency bridge device comprising two coaxial line balancing arms, a third arm of coaxial line construction connected in shunt to said balancing arms and serving as input to said device. and a fourth arm of wave guide construction connected in series to said balancing arms by means of a slot, said fourth arm being the output arm of the bridge, and all of said arms being connected at a common point.

2. An ultra high frequency bridge device comprising two coaxial line balancing arms, a third arm of coaxial line construction connected to said balancing arms in shunt by means of a coaxial T-connection and serving as input to said device, and a fourth arm of wave guide construction connected in series to said balancing arms by means of a slot, said fourth arm being the output arm of the bridge and all of said arms being connected at a common point.

3. An ultra high frequency bridge device as in claim 2, wherein said slot, used to couple said output arm of wave guide construction to said coaxial balancing arms, is resonant, whereby a highly sensitive adjustment of said bridge is accomplished.

4. An ultra high frequency bridge device comprising two coaxial line balancing arms having a common outer conductor, an input arm of coaxial line construction connected in shunt with said balancing arms, and a fourth output arm of wave guide construction coupled in series to said balancing arms by means of a slot made in said outer conductor, said slot being formed at the common connection of said arms.

5. An ultra high frequency device as in claim 4, further including a microwave frequency generator connected to the input arm thereof.

6. An ultra high frequency bridge device as in claim 4, furthercomprising a detector connected to the output arm thereof, whereby the balancing arms may be suitably adjusted to give a null indication at the detector.

7. A high frequency bridge device, comprising, a coaxial transmission line having an inner and an outer conductor, a second coaxial line having an inner conductor connected to said first inner conductor at a junction point intermediate the ends of said first line and also having an outer conductor connected to said first outer conductor, whereby said latter coaxial line is connected effectively in shunt with the two arms provided by the two portions of said first line on either side of said junction point, and a high frequency energy conductor coupled at said junction in symmetrical fashion to both said arms to be responsive solely to the diiference in currents flowing in said two arms of said first line, whereby said two arms provide two balancing arms and said second line and said last-named conductor provide input and output arms for said bridge device.

8. High frequency apparatus as in claim 7. wherein said first outer conductor is provided with a slot opposite said junction point, and wherein said last-named energy conductor comprises a wave guide having an end communicating with said slot.

. 9. -In1combination, a first coaxial line section,

' termediate the ends of said first line, and 9, Write guide connected in: series with said first liniby means of a slot iii the outer conductorof "(1 first line, the twiconnections to said 'first i 'ii ne being at substantially the same location aiiing said first line.

- THEODORE MORENQ.

REFnniaNcEs CITED The following references are of record in the file of this patent;

Number Nuinber 10 I Ti" UNITED sums PA'I'EN'iS Name Date Bowen 0ct. 7, 1941 Salinger Jime 30, 1942 Ring Nov. 12, 1946 Nov. 26, 1946 Hg. 5, 1947 1y 27, 1948 Tyrreli ply 27, 1948 FOREIGN PATENTS Country Date France June 6, 1923 

